ployed the load is usually about
one fourth of that which the animals could actually move by their
maximum effort for a short period. With motor vehicles, the excess
power is provided for by gearing.

If it be assured a load of convenient size is being moved on a level
road by draft animals, there is a limit to the rate of grade up which
the load can be drawn by the maximum effort of the animals.

Tests indicate that the horse can pull at a speed of 2-1/2 miles per
hour, an amount equal to 1/8 to 1/10 of its weight, and for short
intervals can pull 3/4 of its weight. The maximum effort possible is
therefore six times the average pull, but this is possible for only
short intervals. A very short steep hill would afford a condition
where such effort would be utilized. But for hills of any length, that
is, one hundred feet or more but not to exceed five hundred feet, it
is safe to count on the draft animal pulling three times his normal
pulling power for sustained effort.

The limiting grade for the horse drawn vehicle is therefore one
requiring, to overcome the effect of grade, or _f_{g}_, a pull in
excess of three times that exerted on the level.

A team of draft animals weighing 1800 lbs. each could exert a
continuous pull of about 1/10 of their weight or 360 lbs. If it be
assumed that the character of the vehicle and the road surface is such
that _f_{r}_ + _f_{i}_ + _f_{p}_ + _f_{a}_ = 100 lbs. per gross ton on
a level section of road, then the gross load for the team would be 3.6
tons. The same team could for a short time exert an additional pull of
three times 360 lbs. or 1080 lbs. For each 1 per cent of grade a pull
of 20 lbs. per ton would be required or _f_{g}_ for the 3.6 tons load
would be 72 lbs. for each per cent of grade. At that rate, the
limiting grade for the team would be fifteen per cent.

If, however, the character of the vehicle and the road surface were
such that _f_{r}_ + _f_{i}_ + _f_{p}_ + _f_{a}_ = 60 lbs. per gross
ton on a level section of road, the gross load for the team on the
level would be 6 tons, and the limiting grade 9 per cent.

The above discussion serves to illustrate the desirability of adopting
a low ruling or limiting grade for roads to be surfaced with a
material having low tractive resistance and the poor economy of
adopting a low ruling grade for earth roads or roads to be surfaced
with material of high tractive resistance.

It may be questioned whether horse drawn traffi